An SAW convolver receives two input signals S(t) and R(t) and outputs an output signal C(t) given by a following formula: ##EQU1## where t represents the time, T indicates the gate delay time of the convolver, and n is a constant proportional to the convolution efficiency of the convolver.
In almost all the cases the SAW convolver is used usually as a correlator, using the operational function represented by the above formula, and it is applied often in a spread spectrum communication device, a radar, etc.
However, using the operational function represented by the above formula, the SAW convolver can be used also as a band pass filter having a variable central frequency, an AM or FM demodulator, or a simple spectrum analyzer.
FIGS. 6 and 7 indicate prior art examples for realizing the applications described above.
FIG. 6 shows a prior art example, in which an SAW convolver is used as a band pass filter having a variable central frequency, while FIG. 7 shows a prior art example, in which an SAW convolver is used as an AM or FM demodulator or a simple spectrum analyzer.
In FIGS. 6 and 7, reference numeral 1 is an input terminal; 2 and 2' are input matching circuits; 3 is an SAW convolver; 4 and 4' are input transducers; 5 is an output electrode (gate); 6 is an output matching circuit; 7 is an amplifier; 8 is a mixer; 9 is a low pass filter; 10 is a reference signal generator; 11 is an output terminal; and 12 is a detecting circuit. When the frequency pass band of the two input transducers 4 and 4' formed in the SAW convolver is represented by; EQU f.sub.L .about.f.sub.H (f.sub.L &lt;f.sub.H) (1),
the frequency region, which can be matched by the output matching circuit 6, is set at; EQU 2 f.sub.L .about.2 f.sub.H ( 2).
On the other hand the reference signal R(t) outputted by the reference signal generator 10 is a sinusoidal wave of frequency f.sub.r, which is expressed by; EQU R(t)=R.sub.o cos(2.pi.f.sub.r t+.theta..sub.r) (3),
where R.sub.o represents the amplitude; t the time; and .theta..sub.r the initial phase, and the frequency f.sub.r is chosen in a following domain; EQU f.sub.L .ltoreq.f.sub.r .ltoreq.f.sub.H ( 4).
Further, in FIG. 6, a signal obtained by amplifying the output of the convolver inputted in an RF terminal of the mixer 8, while the reference signal R(t) is inputted in an LO (Local) terminal. The output from an IF terminal is subjected to filtering in the low pass filter 9 to form an output E(t).
Here it is supposed that the cut-off frequency of the low pass filter 9 described above is f.sub.H. That is, the filtering is so set that only the frequency component given by; EQU f.ltoreq.f.sub.H ( 5)
is made pass therethrough.
On the other hand, in FIG. 7, the output of the convolver 3 amplified by the amplifier 7 is detected by the detecting circuit 12 and the output after the detection is the output of the whole P(t). Here the detecting circuit 12 is a detecting circuit, which is constructed so as to output a signal proportional to the amplitude of the input signal.
Now, in the prior art SAW filter devices indicated in FIGS. 6 and 7 and constructed as described above, when the input signal S(t) is a sinusoidal wave of frequency f expressed by; EQU S(t)=S.sub.o cos(2.pi.ft) (6),
where S.sub.o represents the amplitude, the output E(t) in FIG. 6 and the output P(t) in FIG. 7 can be expressed as follows; ##EQU2##
In Eq. (7), A is a constant determined by the efficiency of the convolvers 3, the gain of the amplifiers 7 and the efficiency of the mixers 8 and .theta.(f) represents an amount of phase shift determined by phase characteristics of the input and the output matching circuits 2, 2' and 6, the amplifiers 7, the mixer 8 and the low pass filters 9.
On the other hand, in Eq. (8), B is a constant determined by the efficiency of the convolvers 3, the gain of the amplifiers 7 and the efficiency of the detecting circuit 12.
From Eq. (7), it may be obvious that the construction indicated in FIG. 6 can be a band pass filter, whose central frequency is f.sub.r. However, from Eq. (7), amplitude characteristics of the filter can be expressed by; ##EQU3## That is, denoting the difference between the frequency f of the input signal and the frequency f.sub.r of the reference frequency by; EQU x.tbd.f-f.sub.r ( 10),
the amplitude characteristics of the filter indicated in FIG. 6 are passing characteristics proportional to ##EQU4##
On the other hand, as clearly seen from Eq.(8), the output in FIG. 7 has a value, which is also proportional to ##EQU5##
The points described above are explained also e.g. in JP - A - Hei 2-207605 [Reference (1)].
Now, from Eq. (7) and the explanation described above, it may be obvious that the construction indicated in FIG. 6 can be applied as a band pass filter having a variable central frequency (f.sub.r) by varying the frequency f.sub.r of the refrence signal in the construction indicated in FIG. 6.
On the other hand, also in the construction indicated in FIG. 7, when the frequency f.sub.r of the reference signal is swept, using Eq. (8), the intensity of the output signal can be expressed by a function of a form of ##EQU6## whose central frequency is f.sub.r. Therefore it may be obvious that it can be used as a simple spectrum analyzer. Further the construction indicated in FIG. 7 can be used in an AM demodulator or an FM demodulator, utilizing output characteristics of the form of ##EQU7## Concerning details thereof, refer to Reference [1] stated previously.
However the prior art devices indicated in FIGS. 6 and 7 as described above have a problem as follows.
As explained in the description after Eq. (10), it is that the level of side lobes is too high, because both the devices indicated in FIGS. 6 and 7 have passing characteristics or output characteristics of the form of ##EQU8##
FIG. 8 indicates amplitude characteristics .vertline.E(t).vertline. of the filter indicated in FIG. 6 and frequency characteristics of the output P(t) of the construction indicated in FIG. 7, where the abscissa represents (f-f.sub.r)/T and the ordinate shows the value of .vertline.E(t).vertline. or P(t) normalized with respect to the maximum values thereof and f represents the frequency of the input signal; f.sub.r the frequency of the reference signal; and T the gate delay time of the convolver. Here denoting the length of the output electrode of the convolver (gate length) by L and the propagation velocity of the surface acoustic wave by v, a following relation is valid; EQU L=v T (11).
It can be seen from FIG. 8 that the maximum value V.sub.smax of the side lobes S.sub.1 and S.sub.2 with respect to the value V.sub.p of the main lobe corresponding to f=f.sub.r is as great as; ##EQU9## V.sub.smax corresponds to the value when the input frequency f is equal to a frequency f.sub.m (f=f.sub.m) defined by; ##EQU10## From FIG. 8, the band width of the main lobe M is approximately equal to T (width of 3 dB down). It can be understood that, in order to reduce the band width of the filter or to increase the resolving power in the case where it is applied as a spectrum analyzer, it is sufficient to elongate the gate length L. On the other hand, the maximum value of the side lobes S.sub.1 and S.sub.2 is expressed by Eq. (12) and independent of the gate length L.
The value of Eq. (12) indicates that there are problems that the side lobe suppressing ratio is too small, in the case where the construction indicated in FIG. 6 is used as a filter and that the spurious level is too high and it is impossible to have a satisfactorily great dynamic range, in the case where the construction indicated in FIG. 7 is used as a spectrum analyzer. The problems described above cannot be solved, even if the gate length L of the convolver is varied, as described above.
That is, the prior art devices indicated in FIGS. 6 and 7 had a drawback that it was not possible to avoid a problem that the side lobe suppressing ratio was small or the dynamic range was narrow in applications thereof to a filter, a spectrum analyzer, etc. in practice.